The present invention relates generally to determining the surface level of a given body and, more particularly, relates to a method and apparatus for determining the surface level of a given body using acoustic ranging.
Using acoustic ranging to find the level of a liquid or solid body is well known to those practiced in the art. The general approach is to transmit a pulse of sound, listen to the echo, compute the time between transmission and reception, and convert this to a distance by multiplying the time by the speed of sound. An example of present capabilities is embodied in the EZQ River Flow Monitor, manufactured by Nortek AS of Oslo, Norway. The EZQ measures stage (surface level) with a vertical echo sounder that finds the strong echo of the water surface. The surface level or depth can be a very important commodity. For example, it can be very important in determining flow rates of water channels, which can be critical knowledge for a variety of reasons. Often, however, multiple echoes are received due to interference with the acoustic signal by debris. Additionally, multiple bounces of the echo where the depth is shallow can create added echoes, as can echoes generated from strong reflectors outside the acoustic beam or due to imperfect beam design. The additional echoes compete with the surface echo and make it hard to determine which echo actually corresponds to the surface. An issue not addressed by conventional systems is the problem of identifying a specific echo to associate with the surface being measured. There are many circumstances in which spurious echoes compete with the surface echo. The problem is to discern between the desired surface echo and any spurious echoes that may exist.
For example, because there is no control over natural water flows, debris within a flow is a rich source of spurious echoes. At the same time, water level at such sites is an important and economically valuable parameter to measure, as it forms a key element in measuring flow rates. Spurious echoes in rivers can come from debris, bubbles, fish, and plants, to name a few. Spurious echoes also arise from obstacles outside of the main acoustic beam due to imperfect beam design or because the obstacles are particularly strong reflectors.
Properly implemented acoustic sensors will find the surface most of the time. But spurious echoes can introduce spikes and dropouts into the measurement. Cleaning up this noisy data requires human intervention, which increases cost and delays the availability of good data. Consequently, the value of the data for use in automated processes and for automated data reporting is limited. With appropriate visual displays, a human being is easily able to discern echoes from the water surface and to filter out spurious echoes. This is because a human being is able to detect patterns and history within the information displayed, can include prior knowledge of measurement results, and can weigh evidence within the real world and within the results obtained. What is needed is an automated apparatus and method that incorporates processes natural to human beings, to improve identification of the surface echo in environments in which it might otherwise be overshadowed by spurious echoes.
The present invention is directed toward a method and apparatus for determining the surface level of a given body using acoustic ranging. In order to determine the surface level of a given body, whether the body is a solid or a liquid, an acoustic pulse is sent through the body. The pulse bounces off the surface of the particular body being investigated and returns toward the source, thus forming an echo. When the echo arrives back at the source, it is received and processed into data relating the strength of the echo and the roundtrip time from transmission to reception. The time is then converted into a distance by multiplying the time by the speed of sound within the body.
The surface level or depth can be a very important commodity. For example, it can be very important in determining flow rates of water channels, which can be critical knowledge for a variety of reasons. Often, however, multiple echoes are received due to interference with the acoustic signal by debris. Additionally, multiple bounces of the echo where the depth is shallow can create added echoes, as can echoes generated from strong reflectors outside the acoustic beam or due to imperfect beam design. The additional echoes compete with the surface echo and make it hard to determine which echo actually corresponds to the surface. The claimed invention overcomes this problem and enables an instrument to automatically select the echo corresponding to the surface reflection.
As such, a method for determining the surface level of a given body using acoustic ranging is presented. First, an acoustic pulse is transmitted through the liquid or solid body. In one embodiment, an electric signal is converted into an acoustic pulse, which is transmitted in an upward direction through the solid or liquid. The acoustic pulse travels vertically toward the surface where it is reflected. Any debris within the path of the pulse, as well as strong reflectors outside the path, may also reflect the acoustic pulse. For example, if the body is a liquid body in a channel, the pulse may reflect off silt, or other debris traveling in the channel. Additionally, imperfect beam design may allow generation of spurious echoes and, in shallow bodies, the echo may bounce up and down multiple times creating multiple spurious echoes.
As a result, the echo from the surface as well as spurious echoes is received. In one embodiment, the surface echo and multiple spurious echoes are converted into electric receive signals. The received signals are then filtered and processed for analysis and display to a user. The processed signals are then evaluated by locating peaks within the data, which represent strong echoes that may correlate to the surface echo. The peaks are then evaluated according to a variety of criteria to arrive at a measurement of the quality of each peak. The higher the quality, the more likely the peak represents the surface echo, as opposed to a strong spurious echo. Finally, the peak with the highest quality measurement is determined to represent the surface echo and the roundtrip time associated with the peak is converted to a distance representing the depth of the body.
In one embodiment, the evaluation criteria involves measuring the amplitude of the received echo, in order to determine the echo""s signal strength, measuring the signal to noise ratio of the echo, and measuring the width of the received echo. These measurements are then converted into a quality measurement for the echo. The higher the quality measurement, the more likely the echo corresponds to the surface. Sometimes, however, the surface echo may be weaker than a particularly strong spurious echo or echoes, or the surface echo may not be present at all. Therefore, additional evaluation criteria may be required to select the correct echo or to enable ignoring a particular echo.
For example, in one embodiment, a first echo is looked at in relation to subsequent echoes to determine if the subsequent echoes are multiple bounces of the first echo. This can occur when the depth of the body is relatively shallow. The transmitted acoustic pulse will bounce off the surface and return to the source with relatively strong signal strength. When it arrives back at the source, it is reflected back toward the surface and the process starts over. The result of this phenomenon is the reception of several echoes evenly spaced in time. By looking at the time relationship of subsequent echoes with respect to a first echo, it can be determined if the subsequent echoes are multiple bounces of the first. The subsequent echoes that appear to be multiple bounces of a first echo can then be given lower quality measurements to account for this likelihood.
In one embodiment, an independent input device is used to supply a rough estimate of the surface level. Received echoes are then evaluated for how closely they correspond to the measurement provided by the independent input. For example, in one implementation used to determine the surface level of a liquid body, a pressure sensor can be used to perform the independent measurement. In another implementation, a human being enters the approximate water level manually. Echoes that are relatively close to the measurement provided by the pressure sensor are then afforded a higher quality measurement, as they are more likely to be the surface echo.
In another embodiment, various data are combined mathematically in order to provide a better first estimate of the surface location, as opposed to the rough estimate provided by a pressure sensor alone, in situations where water level moves up and down as a result of surface waves. In such cases, instantaneous pressure consists of a mean pressure signal plus a varying pressure signal that varies at the frequency of the waves. Waves introduce errors associated with the fact that the varying pressure signal attenuates with depth. A better first estimate of the instantaneous water level can, in such cases, be obtained with a mathematical equation that combines parameters such as the mean and varying pressure signal and its time derivative, and the vertical velocity and its time and depth derivatives.
In another embodiment, a distribution is generated from historical data comprising measurements of the surface level. In one implementation, the distribution is weighted in favor of more recent history. Echoes that fall within a high distribution are then afforded a higher quality measurement than those that fall within lower distributions.
There is also provided an apparatus for determining a surface level of a given body using acoustic ranging. The device comprises a transducer for transmitting acoustic signals and receiving echoes of transmitted acoustic signals, and transmit electronics for applying an electric signal to the transducer. In one embodiment, the transducer converts the electric signal into the acoustic transmission. The device also includes receive electronics, wherein the transducer converts the acoustic echoes into electric signals and the receive electronics filter and condition the electric signals for analysis. A processor is also included for computing and digitizing the electric signals. Additionally, an independent measuring device is included for measuring the surface level of the body. In one implementation, the independent measuring device is a pressure sensor.
There is also provided a device for determining a surface level of a given body using acoustic ranging, comprising a storage means for storing firmware that is used to run the device and for storing data that is collected by the device. The device also comprises an input/output interface for uploading the firmware from a personal computer or the like, and for downloading the collected data to a personal computer or external storage device. A transducer is also included for transmitting acoustic signals and receiving echoes of transmitted acoustic signals. Additionally, a pressure sensor is included for performing independent measurements of the surface level. The device also comprises a processor for controlling the operation of the device, including the operation of the transducer, the pressure sensor, the input/output interface, and the storing of the collected data. The processor controls the device operation by running the firmware stored in the storage means. Finally, a power supply is included that interfaces to and conditions the voltage supplied by batteries used to power the device.